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Showing posts from January, 2008

An Almost Type-Safe General Monad in C#, aka how to Abstract over Type Constructors using Dynamics

Extending the work in my last post, I've developed a way to express an almost type-safe, general monad in C#. Similar to my module translation, the single monad object becomes a pair of co-operating objects, only one of which the monad implementor must define. Since C# cannot abstract over type constructors, I had to exploit the only feature that could accomodate the flexibility I needed: C#'s dynamic typing.
// The Monad object, indexed by a singleton type that implements the
// monad operations.
public sealed class Monad<M, T>
where M : struct, IMonadOps<M> { ... }

// An object that implements operations on the monad's encapsulated
// state.
public interface IMonadOps<M>
where M : struct, IMonadOps<M>
{
/// Return the encapsulated state for the monad's zero value.
object Zero<T>();

// Return the encapsulated state for the 'unit' operation.
object Unit<T>(T t);

// Perform a bind operation given the monad's e…

The Worst Monad Tutorial... Except For All Those Others.

I've found other monad tutorials very frustrating. They are typically written in expressive languages with type inference, which permits concise descriptions, but obscures the underlying type structure. I've been struggling with writing something close to a monad in C# for quite some time, simply because none of these tutorials give a sufficiently complete description of a monad's structure. Suprisingly, the Wikipedia page on monads helped clarify what I was missing.

Here is the general structure of a monad all these tutorials use:
-- the type of monad m
type m a = ...

-- return is a type constructor that creates monad instances
return :: a → m a

-- bind is a function that combines a monad instance m a with a
-- computation that produces another monad instance m b from a's
-- to produce a new monad instance m b
bind :: m a → (a → m b) → m b
So the monad type 'm', has a function 'return' that constructs instances of that type, and 'bind' which converts a…

Towards the best collection API... in C#. And some partial applications too.

The venerable Oleg Kiselyov once posted about the "best" collection traversal API. Let's call this ideal iterator a "SuperFold". LTU also covered his article. Essentially, a SuperFold is a left fold with early termination support. Any cursor can then be automatically derived from the SuperFold. The converse is not true.

Additional arguments are made in the above paper and in the LTU thread, so I won't repeat them here. Without further ado, I present the SuperFold for my purely functional list in C#:
//OCaml signature: ((T → B → bool * B) → B → B) → (T → B → bool * B) → B → B
B SuperFold<B>(
Fun<Fun<T, B, Pair<bool, B>>, B, B> self,
Fun<T, B, Pair<bool, B>> proc,
B seed)
{
bool cont;
proc(head, seed).Bind(out cont, out seed);
return cont ? self(proc, seed) : seed;
}
While quite simple, it's not as efficient as it should be since C#/.NET doesn't support proper tail calls. You can see in that source file that I deriv…

ML Modules in C# - Sorely Missing Polymorphic Type Constructors

As Chung-chieh Shan pointed out, my encoding of modules in C# is somewhat limited. In particular, I cannot abstract over type constructors, which is to say, C# is missing generics over generics. Consider the Orc.NET interpreter:
class Orc {
class Exp<T> { ... }

public Exp<U> Seq<T,U>(Exp<T> e1, <U>) { ... }
public Exp<T> Par<T>(Exp<T> e1, Exp<T>) { ... }
public Exp<T> Where<T>(Exp<T> e1, Exp<Promise<T>>) { ... }
}
This is the result of my translation, which was necessitated by the "Where" method. Where introduces a dependency which currently cannot be expressed with ordinary C# constraints, so the module encoding is necessary.

The above interface is a direct, faithful implementation of the Orc semantics. The implementation I currently have is an interpreter for those semantics. What if I want to provide a compiler instead? The interface should remain the same, but the implementation should di…